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About Thorium

Thorium Bohr

Thorium is a relatively common radioactive element in the actinide family. In pure form, it is a soft silvery-white metal with properties very similar to lead, and can be shaped rather easily under proper conditions. The abundance of thorium within the earth’s crust is much greater (15 ppm) than other radioactive elements, particularly uranium. This makes thorium an attractive candidate for lowering the cost of nuclear power sources in the future. Thorium is typically isolated by converting a sample of monazite or thorite to thorium dioxide, and then heating the compound with calcium; which, if desired (e.g. breeder reactors), can then be transformed into various synthetic isotopes of uranium through neutron bombardment. Only one thorium nuclear reactor was ever built (1979) and due to numerous economic and technical problems, shut down after only a decade.

The native characteristics of thorium lend itself to a number of compounds with industrial and consumer applications. In fact, thorium is the only radioactive element besides uranium with non-radioactive applications. Central to these applications is its ability to maintain strength at high temperature. In some magnesium alloys, thorium is used in aircraft engines and rockets. Oxidized thorium is used in arc welders. Compounds such as thorium dioxide and thorium nitrate are used in various lights and lamps as a brilliant white light source when heated with a gas flame. Thoriated tungsten elements are contained in filaments of magnetrons, most commonly exemplified in microwave ovens and radar systems. Furthermore, thorium dioxide is utilized in many ceramics and glasswork applications. Thorium dioxide is known to increase refractive index in glass, thus has many applications in optical lenses for cameras and scientific apparatuses. Thorium fluoride is used as an optical coating, and the list goes on. Although exposure to small amounts of thorium in applications such as those listed above is considered safe, thorium-based products have begun to fall out of favor (2000s) due to its perceived biological danger.

The element was discovered in 1828 by Swedish chemist Jons Jakob Berzelius, at a time when the concept of radioactivity was not yet known.  It was first recognized as a radioactive element as a result of English chemist Gerhard C. Schmidt and Marie Curie’s radioactivity research in 1898.  There are 27 known isotopes of thorium, all radioactive.  The isotope with the longest half-life is 232Th, at a staggering 14 billion years, which explains this primordial radioactive element’s relative abundance on Earth.  This particular isotope decays into 228Ra through alpha decay; other elemental decay products of thorium isotopes include radon and actinium.


Thorium is a lanthanide (rare earth) material with potential nuclear power applications. It is presently used as a tungsten coating in electronic parts due to its high emission factor. In its fluoride and oxide forms, Thorium is used in advanced optic applications for its high refractive index. It is also used in several other high temperature glass applications, such as in the mantle of lamps and to produce crystal growth crucibles and ampules. High Purity (99.999%) Thorium (Th) Sputtering TargetThorium is available as metal and compounds with purities from 99% to 99.999% (ACS grade to ultra-high purity). High Purity (99.999%) Thorium Oxide (ThO2) Powder Oxides are available in powder and dense pellet form for such uses as optical coating and thin film applications. Oxides tend to be insoluble. Fluorides are another insoluble form for uses in which oxygen is undesirable such as metallurgy, chemical and physical vapor deposition and in some optical coatings. Thorium is also available in soluble forms including chlorides, nitrates and acetates. These compounds can be manufactured as solutions at specified stoichiometries.

Thorium Properties

Thorium(Th) atomic and molecular weight, atomic number and elemental symbol Thorium Bohr ModelThorium is a Block F, Group 3, Period 7 element. The number of electrons in each of Thorium's shells is 2, 8, 18, 32, 18, 10, 2 and its electron configuration is [Rn] 6d2 7s2. The thorium atom has a radius of and its Van der Waals radius is In its elemental form, CAS 7440-29-1, thorium has a silvery, sometimes black-tarnished, appearance. Elemental Thorium Thorium was first discovered by Jons Berzelius in 1829. It is found in small amounts in most rocks and soils. The name Thorium originates from the Scandinavian god Thor, the Norse god of war and thunder.

Symbol: Th
Atomic Number: 90
Atomic Weight: 232
Element Category: Actinide
Group, Period, Block: n/a, 7, f
Color: silvery white
Other Names: N/A
Melting Point: 1842 °C, 3348 °F, 2115 K
Boiling Point: 4788 °C, 8650 °F, 5061 K
Density: 11.7 g·cm3
Liquid Density @ Melting Point: N/A
Density @ 20°C: 11.7 g/cm3
Density of Solid: 11724 kg·m3
Specific Heat: 0.13 kJ/kg/K
Superconductivity Temperature: 1.38 [or -271.77 °C (-457.19 °F)] K
Triple Point: N/A
Critical Point: N/A
Heat of Fusion (kJ·mol-1): 19.2
Heat of Vaporization (kJ·mol-1): 513.67
Heat of Atomization (kJ·mol-1): 598.65
Thermal Conductivity: 54.0 W·m-1·K-1
Thermal Expansion: (25 °C) 11.0 µm·m-1·K-1
Electrical Resistivity: (0 °C) 147 nΩ·m
Tensile Strength: 144 MPa
Molar Heat Capacity: 26.230 J·mol-1·K-1
Young's Modulus: 79 GPa
Shear Modulus: 31 GPa
Bulk Modulus: 54 GPa
Poisson Ratio: 0.27
Mohs Hardness: 3
Vickers Hardness: 350 MPa
Brinell Hardness: 400 MPa
Speed of Sound: (20 °C) 2490 m·s-1
Pauling Electronegativity: 1.3
Sanderson Electronegativity: N/A
Allred Rochow Electronegativity: 1.11
Mulliken-Jaffe Electronegativity: N/A
Allen Electronegativity: N/A
Pauling Electropositivity: 2.7
Reflectivity (%): N/A
Refractive Index: N/A
Electrons: 90
Protons: 90
Neutrons: 142
Electron Configuration: [Rn] 6d2 7s2
Atomic Radius: 179 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: 206±6 pm
Covalent Radius (Å): 1.9
Van der Waals Radius: 237 pm
Oxidation States: 4, 3, 2, 1 (weakly basic oxide)
Phase: Solid
Crystal Structure: face-centered cubic
Magnetic Ordering: paramagnetic
Electron Affinity (kJ·mol-1) Unknown
1st Ionization Energy: 587 kJ·mol-1
2nd Ionization Energy: 1110 kJ·mol-1
3rd Ionization Energy: 1930 kJ·mol-1
CAS Number: 7440-29-1
EC Number: 231-139-7
MDL Number: N/A
Beilstein Number: N/A
SMILES Identifier: [Th]
InChI Identifier: InChI=1S/Th
PubChem CID: 23960
ChemSpider ID: 22399
Earth - Total: 51.2 ppb 
Mercury - Total: 39.4 ppb
Venus - Total: 53.7 ppb
Earth - Seawater (Oceans), ppb by weight: 0.00004
Earth - Seawater (Oceans), ppb by atoms: 1.1E-06
Earth -  Crust (Crustal Rocks), ppb by weight: 6000
Earth -  Crust (Crustal Rocks), ppb by atoms: 540
Sun - Total, ppb by weight: 0.3
Sun - Total, ppb by atoms: 0.002
Stream, ppb by weight: 0.1
Stream, ppb by atoms: 0.0004
Meterorite (Carbonaceous), ppb by weight: 40
Meterorite (Carbonaceous), ppb by atoms: 3
Typical Human Body, ppb by weight: N/A
Typical Human Body, ppb by atom: N/A
Universe, ppb by weight: 0.4
Universe, ppb by atom: 0.002
Discovered By: Jöns Jakob Berzelius
Discovery Date: 1829
First Isolation: N/A

Health, Safety & Transportation Information for Thorium

Thorium is radioactive and can collect in bones which may cause bone cancer several years after exposure. Breathing in substantial amounts of thorium may be lethal. Safety data for Thorium and its compounds can vary widely depending on the form. For potential hazard information, toxicity, and road, sea and air transportation limitations, such as DOT Hazard Class, DOT Number, EU Number, NFPA Health rating and RTECS Class, please see the specific material or compound referenced in the Products tab. The below information applies to elemental (metallic) Thorium.

Safety Data
Material Safety Data Sheet MSDS
Signal Word Danger
Hazard Statements H272-H301-H373-H315-H319-H335
Hazard Codes N/A
Risk Codes N/A
Safety Precautions N/A
RTECS Number XO6400000
Transport Information UN 2912
WGK Germany 3
Globally Harmonized System of
Classification and Labelling (GHS)
Exclamation Mark-Acute Toxicity Oxidizing Liquid - Oxidizing GasHealth HazardSkull and Crossbones-Acute Toxicity

Thorium Isotopes

Thorium (Th) has six naturally occurring isotopes. None of these are stable; however, 232Th is observationally stable with a half life of 14.05 billion years.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
209Th 209.01772(11) 7(5) ms [3.8(+69-15)] Unknown 5/2-# N/A 1578.15 -
210Th 210.015075(27) 17(11) ms [9(+17-4) ms] a to 206Ra 0+ N/A 1586.23 -
211Th 211.01493(8) 48(20) ms [0.04(+3-1) s] a to 207Ra 5/2-# N/A 1594.31 -
212Th 212.01298(2) 36(15) ms [30(+20-10) ms] a to 208Ra; ß+ to 212Ac 0+ N/A 1602.38 -
213Th 213.01301(8) 140(25) ms a to 209Ra 5/2-# N/A 1610.46 -
214Th 214.011500(18) 100(25) ms a to 210Ra 0+ N/A 1618.54 -
215Th 215.011730(29) 1.2(2) s a to 211Ra (1/2-) N/A 1626.62 -
216Th 216.011062(14) 26.8(3) ms a to 212Ra; ß+ to 216Ac 0+ N/A 1634.7 -
217Th 217.013114(22) 240(5) µs a to 213Ra (9/2+) N/A 1642.78 -
218Th 218.013284(14) 109(13) ns a to 214Ra 0+ N/A 1650.86 -
219Th 219.01554(5) 1.05(3) µs a to 215Ra 9/2+# N/A 1658.93 -
220Th 220.015748(24) 9.7(6) µs a to 216Ra; EC to 220Ac 0+ N/A 1667.01 -
221Th 221.018184(10) 1.73(3) ms a to 217Ra (7/2+) N/A 1675.09 -
222Th 222.018468(13) 2.237(13) ms a to 218Ra; EC to 220Ac 0+ N/A 1683.17 -
223Th 223.020811(10) 0.60(2) s a to 219Ra (5/2)+ N/A 1681.93 -
224Th 224.021467(12) 1.05(2) s a to 220Ra; 2ß+ to 225Ac 0+ N/A 1690.01 -
225Th 225.023951(5) 8.72(4) min a to 221Ra (3/2)+ N/A 1698.09 -
226Th 226.024903(5) 30.57(10) min a to 222Ra 0+ N/A 1706.17 -
227Th 227.0277041(27) 18.68(9) d a to 223Ra 1/2+ N/A 1714.25 -
228Th 228.0287411(24) 1.9116(16) y a to 224Ra; 20O 0+ N/A 1722.33 -
229Th 229.031762(3) 7.34(16)E+3 y a to 225Ra 5/2+ 0.46 1721.09 -
230Th 230.0331338(19) 7.538(30)E+4 y a to 226Ra; SF 0+ N/A 1729.17 -
231Th 231.0363043(19) 25.52(1) h a to 227Ra; ß- to 231Pa 5/2+ N/A 1737.25 -
232Th 232.0380553(21) 1.405(6)E+10 y a to 228Ra; SF 0+ N/A 1745.33 100
233Th 233.0415818(21) 21.83(4) min ß- to 233Pa 1/2+ N/A 1744.09 -
234Th 234.043601(4) 24.10(3) d ß- to 234Pa 0+ N/A 1752.17 -
235Th 235.04751(5) 7.2(1) min ß- to 235Pa (1/2+)# N/A 1760.25 -
236Th 236.04987(21)# 37.5(2) min ß- to 236Pa 0+ N/A 1768.32 -
237Th 237.05389(39)# 4.8(5) min ß- to 237Pa 5/2+# N/A 1767.09 -
238Th 238.0565(3)# 9.4(20) min ß- to 238Pa 0+ N/A 1775.16 -